A variety of techniques and apparatus have been used to satisfy the requirements of automated currency handling systems. At the lower end of sophistication in this area of technology are systems capable of handling only a specific type of currency, such as a specific dollar denomination, while rejecting all other currency types. At the upper end are complex systems which are capable of identifying and discriminating among multiple currency denominations.
Currency discrimination systems typically employ either magnetic sensing or optical sensing for discriminating between different currency denominations. Magnetic sensing is based on detecting the presence or absence of magnetic ink in portions of the printed indicia on the currency by using magnetic sensors, usually ferrite core-based sensors, and using the detected magnetic signals, after undergoing analog or digital processing, as the basis for currency discrimination. The more commonly used optical sensing technique, on the other hand, is based on detecting and analyzing variations in light reflectance or transmissivity characteristics occurring when a currency bill is illuminated and scanned by a strip of focused light. The subsequent currency discrimination is based on the comparison of sensed optical characteristics with prestored parameters for different currency denominations, while accounting for adequate tolerances reflecting differences among individual bills of a given denomination.
A major obstacle in implementing automated currency discrimination systems is obtaining an optimum compromise between the criteria used adequately define the characteristic pattern for a particular currency denomination and the time required to analyze test data and compare it to predefined parameters in order to identify the currency bill under scrutiny. Even with the use of microprocessors for processing the test data resulting from the scanning of a bill, a finite amount of time is required for acquiring samples and for the process of comparing the test data to stored parameters to identify the denomination of the bill.
Some of the optical scanning systems available today employ two optical scanheads disposed on opposite sides of a bill transport path. One of the optical scanheads scans one surface (e.g., green surface) of a currency bill to obtain a first set of reflectance data samples, while the other optical scanhead scans the opposite surface (e.g., black surface) of the currency bill to obtain a second set of reflectance data samples. These two sets of data samples are then processed and compared to stored characteristic patterns corresponding to the green surfaces of currency bills of different denominations. If degree of correlation between either set of data samples and any of the stored characteristic patterns is greater than a predetermined threshold, then the denomination of the bill is positively identified.
A drawback of the foregoing technique for scanning both surfaces of a currency bill is that it is time-consuming to process and compare both sets of data samples for the scanned bill to the stored characteristic patterns. The set of data samples corresponding to the black surface of the scanned bill are processed and compared to the stored characteristic patterns even though no match should be found. As previously stated, the stored characteristic patterns correspond to the green surfaces of currency bills of different denominations.
Another drawback of the foregoing scanning technique is that the set of data samples corresponding to the black surface of the scanned bill occasionally leads to false positive identification of a scanned bill. The reason for this false positive identification is that if a scanned bill is slightly shifted in the lateral direction relative to the bill transport path, the set of data samples corresponding to the black surface of the scanned bill may sufficiently correlate with one of the stored characteristic patterns to cause a false positive identification of the bill. The degree of correlation between the set of "black" data samples and the stored "green" characteristic patterns should, of course, not be greater than the predetermined threshold for positively identifying the denomination of the bill.